In recent years, 2.5 GHz in the coming generation multimedia mobile communication and 2 to 30 GHz in a wireless LAN are employed in a high-frequency band in the communication field. Also, 5.8 GHz in an ETS (electronic toll system) and 76 GHz in an AHS (advanced cruise-assist highway system) are used in a high-frequency band in an ITS (intelligent transport system). In the future, it is expected that fields in which the high-frequency band is applied are further spread rapidly.
Also, a communication terminal represented by a cellular phone has been downsized or brought into a built-in module, and the necessity of an antenna that is high in efficiency and small in size is rising in high-frequency receiving and transmitting portions adaptive to a broad band.
What are widely used as an antenna for a mobile radio device such as the cellular phone are a fixed helical antenna and a built-in type planar inverted-F antenna. In the fixed helical antenna, a fixed helical antenna element is disposed to realize an antenna system that is small in size and light in weight. Also, in the planar inverted-F antenna, radiation elements are so arranged as to be close to each other in parallel with a radio device substrate. A part of the radiation elements is grounded to an earth point, and an electricity is fed to another part of the radiation elements from an electricity feeding point, to thereby realize a short antenna and enable the design of a cellular phone in which the antenna is not projected from a cellular phone body.
However, even in both the cases of the fixed helical antenna and of the planar inverted-F antenna, the communication band of the antenna element is specified and therefore it is impossible to maintain the efficiency over plural bands. Also, in the case where the communication terminal is brought into a module, the size of the antenna portion becomes larger than the size of a circuit chip, and the downsizing of the module is restricted.
Accordingly, an object of the present invention is to provide an antenna which is excellent in the response characteristic in a high-frequency band and very small in size. Also, another object of the present invention is to provide a communication device that has a transmitting function, a receiving function, or a transmitting/receiving function, which includes an antenna that is excellent in the response characteristic in the high-frequency band and very small in size, and which can realize the downsizing of the overall device.